The present invention relates to a process for producing phthalic acid esters. More particularly, the present invention pertains to a new and improved process for the preparation of phthalic acid esters of the following formula (I) in high yield: ##STR2## wherein R and R.sub.1 each is a straight or branch chain aliphatic alkyl group containing 4 to 13 carbon atoms and R and R.sub.1 are the same or different from each other. The esters are made from phthalic acids of the following formula (II): ##STR3## which are reacted with aliphatic alcohols in the presence of a catalyst.
In the above formulas (I) and (II), the various groups are positioned on the ring either ortho, meta or para with respect to each other.
The phthalic acid esters of the present invention are useful with polymeric materials, particularly as plasticizers for PVC. The dioctylphthalate of the phthalic acid esters of the present invention is particularly useful as an additive for inks, paints, adhesives and the like. When used in this capacity it is present in an amount of up to about 40% by weight. The plasticizers of the present invention improve the physical properties of the polymers, such as the heat resistance, cold resistance, and electronic activity of the polymers simultaneously with low volatility or processing activity.
Various processes for the preparation of phthalic acid esters are known in the art. Specific examples of such processes are as follows:
(1) A process for the preparation of phthalic acid esters wherein the catalyst which is utilized is sulfuric acid for reducing corrosion and production costs. This process, however, has a number of disadvantages. Thus, by-products such as ethers or olefins are produced by the dehydration of the alcohols when the concentration of the sulfuric acid is increased or the reaction temperature becomes elevated.
Furthermore, there are the disadvantages of a low yield and a dark colored product which is more than 7 times the color density of the phthalic esters. It is also described in this process that high reaction speed, high yield and low color density can be obtained using metal catalysts such as alumina, tin, lead and zinc. However, utilizing these catalysts creates an energy problem because the reaction temperature has to be elevated higher than the boiling point of the alcohols and it takes about 7 hours to complete the reaction.
(2) The Encyclopedia of Chemical Technology Interscience, New York, 1965 discloses a process for producing phthalic acid esters in the presence of an organic compound catalyst such as titanium or zirconium. However, this process will be difficult to industrialize because of the high catalyst cost and the difficulty in its recovery.
(3) British Pat. No. 1,061,172 discloses a process for the production of carboxylic acid esters which have been catalyzed by titanium peroxide, for example, hydrated or partially hydrated titanium peroxides. Advantages such as a high yield, a short reaction time, good color density and multiple use of the catalyst are features of this process. However, this process cannot be industrialized because of the high cost of the catalyst.
(4) French Pat. No. 1,529,507 discloses a process for the production of phthalic acid esters without the use of a catalyst to eliminate the neutralization and washing steps. This patent, however, has the disadvantages of high equipment costs and high power requirements when using three stage reactors at high temperatures of 210.degree., 215.degree. and 220.degree. C.
All of the prior art processes described above are limited to the orthodioctylphthalate, in contrast to the present invention.
Recently, improvements have been made in this process of esterification for eliminating or resolving the disadvantages or problems noted hereinabove. Thus, the process has been divided into the 5 steps of reaction neutralization, washing, alcohol recovery, reduced pressure distillation and discoloration. However, a large amount of time is required for the neutralization and wash step after the reaction is completed which is the reason why sulfuric acid or para-toluene sulfuric acid is used as the catalyst. Recently, an organic tin compound has been used as the catalyst in order to eliminate the neutralization step. However, this catalyst has the disadvantages of a high reaction temperature, cloudy color, high acidity of the product and the high cost of equipment and power. Accordingly, it is desirable to develop an improved process for producing phthalic acid esters in excellent yield utilizing low reaction temperatures, and eliminating the need of a neutralization or wash step with easy catalyst recovery.